Major Thunderstorm Event Continues

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An unusually deep offshore cutoff low that has tapped a 1 – 1.3″ sub-tropical/monsoonal moisture plume from the southwest and northern Mexico will continue to spiral waves of energy around the central low and spark several rounds of thunderstorms over the days to come until the low finally pushes inland later Wednesday and exits to the east.

Offshore cutoff low with overlayed 300mb heights and surface dew points via metar sites.

Offshore cutoff low with overlayed 300mb heights and surface dew points via metar sites.

Monday afternoon mountain storms will become widespread once again, throughout the sierra, northern mountains, and coastal mountains. In the late afternoon and evening, a wave of thunderstorms over the southern/central sierra may shift westward into the northern San Joaquin valley and travel northwest, possible becoming stronger as it heads further north. HRRR indicates fairly strong instability overnight from around Highway 50 northward, including surface CAPE values in excess of 500 – 1000j/kg and possibly MLCAPEs in the same ballpark as well. Effective shear between 10 – 25kts may allow for some stronger elevated thunderstorms overnight in combination with the decent instability and cool upper level temperatures (-10c offshore with the core of the low).

Nocturnal thunderstorms Monday night may continue into Tuesday morning before new storms form Tuesday afternoon. The same nocturnal events could occur Tuesday night and again Wednesday night, however the threat will shift northward each night. Even though generally the threat will shift northward each day, there could still be isolated storms anywhere across northern California.

Cutoff low to bring several rounds of thunderstorms over the next several days.

Cutoff low to bring several rounds of thunderstorms over the next several days.

Thunderstorms will be capable of many things over the next few days. With there being a pretty strong/deep offshore cutoff low, northern California remains in the entrance region of the lows jet, which tops out between 60 and 70kts. A split off jet of this strength increases divergence aloft, which in combination with decent instability, moisture, and shear easily supports convection. Thunderstorms will be elevated in this event, as there is a fairly strong low level inversion hovering around 900mb. But above this inversion lies some steep lapse rates, in excess of 8c in some cases. The steep lapse rates and elevated instability, and shear available will support thunderstorms capable of strong winds, most likely via outflow, microbursts, and ownbursts in most cases, though some thunderstorm complexes could produce some bowing segments. Hail is also very well possible, possibly severe in some cases in thunderstorms that have stronger updrafts to loft hail well above the freezing level.

12z NAM 300mb heights an winds valid for Tuesday morning.

12z NAM 300mb heights an winds valid for Tuesday morning.

With precipitable water values over an inch (possibly near 1.5″ locally), heavy rainfall directly under thunderstorms is a good bet. One issue with this setup is that the surface and low levels are fairly dry, and will take some time to moisten up enough for precipitation to hit the ground. Even if precipitation does hit the ground, these types of thunderstorms (elevated) are known to produce significant amounts of lightning. Not all of the lightning is likely to occur directly under storms, which equates to dry strikes and possible fire starts. Near thunderstorms, winds could be gusty and/or erratic… especially near decaying storms where outflow, downbursts, or microbursts could carry down some quite strong or possibly severe given 60 – 70kt winds between 500 – 300mb. Gusty winds after a fire start could lead to rapid fire growth given the very dry brush across California.

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